1. Field of the Invention
The present invention relates to a superconducting solenoid, and more particularly, to an insulating structure for such a superconducting solenoid which is capable of improving superconducting stability and electrical insulation.
2. Description of the Prior Art
FIG. 3 is a cross sectional view of a superconducting solenoid, generally designated by reference numeral 100, made in accordance with a conventional "wind and react" procedure, which is described in literature such as, for example, in a paper entitled "High-Field Magnet Formed of New Nb.sub.3 Sn Wires", by Koizumi et al., issued in May 1978 in the preprint of the Twentieth Meeting of the Cryogenic Association of Japan.
In this Figure, the superconducting solenoid 100 includes a winding frame or core 101 in the form of a cylinder and a coil winding 102 which, as clearly illustrated in FIG. 4, is made by winding wires 103 of filamentary conductors around the winding frame 101, each of the wires being covered with an electrical insulator 104 formed of a heat-resisting material such as glass fibers. The wires 103 thus wound around the winding frame 101 are heat treated to provide superconductivity, and a resinous material 105 is impregnated between turns in the winding so as to obtain a sturdy winding construction.
Now, a conventional coil-making procedure will be described. First, wires of filamentary conductors formed of unreacted metal composite are prepared which are each covered with an insulator formed of an electrically insulating material. For such an electrically insulating material, glass fibers having heat resistance and formed into a yarn are chosen. In this connection, however, it is to be noted that since there are various kinds of glass fibers ranging from low grades to high grades, glass fibers, generally called E glass, S glass or the like, are employed which have a melting point higher than about 850.degree. C. To improve workability and stability of the glass fibers, binders such as starch are added in the smallest possible quantities. The wires thus covered with the insulator of glass fibers are wound around a winding frame or core and then heat treated or fired at a temperature of about 800.degree. C. to produce Nb.sub.3 Sn, thus making a superconducting solenoid.
The superconducting solenoid in this state, however, has a loose winding structure and can not operate in an appropriate manner. This is because clearances formed between the coil windings permit the wires of filamentary conductors to move relative to each other under the action of magnetic field created upon energization of the solenoid so that superconductivity of the solenoid will collapse due to frictional heat generated by mechanical contact of neighboring turns of wires and/or generation of heat caused by electromagnetic forces. In order to prevent such a situation, it is ordinary practice to impregnate a resinous material between turns in the winding, as illustrated in FIG. 4, thereby ensuring superconducting stability.
With the conventional superconducting solenoid as constructed in the above-described "wind and react" procedure, the component of starch contained in the glass fibers decomposes during heat treatment with the result that carbon thus decomposed remains sedimented in and adhered to the coil windings, considerably reducing the insulating resistance and hence creating reliability problems in the operation of the entire solenoid. Otherwise, even though it is possible for the solenoid to operate properly in practice, it becomes difficult to detect defects, such as short-circuits of the coil windings which may occur during the production process of the solenoid, by measuring voltage drops or the inductance of the solenoid since if the insulating resistance of the entire solenoid is low, current flows across the adjacent turns of the coil windings to produce the same phenomenon as in short circuits.
In addition, in case where it is necessary to produce intermetallic compounds at a temperature higher than that required for Nb.sub.3 Sn, the glass fibers may melt at such high temperatures to produce short-circuits between the adjacent turns of the coil winding and thus can not provide any satisfactory electrical insulation.